The present invention relates to a hydrometric vane or so-called sail wheel of Waltmann.
European patent document EP-PS 0,076,189 Bl shows a flow measuring device or flow meter of the above mentioned type. The device disclosed in this reference has a guiding element for passing a fluid medium to be measured under a predetermined pressure, and a turbine located in the guiding element and having vanes on a periphery of its cylindrical hub. The turbine is supported rotatably and also axially movably between two stationary casings located upstream and downstream respectively. Also a means is provided to produce a zone with an underpressure of approximately constant value independently from the axial position of the hub at one side of a transverse surface of the upstream end of the hub. On the other hand, the downstream casing embraces a hollow cylindrical part in which the downstream end of the above mentioned hub extends with a play to provide an overpressure zone at the side of the transverse surface of the downstream end of the hub. Thereby a hydrodynamic thrust, resulting from the flow of the fluid medium, is always compensated due to the difference between the above mentioned overpressure and the above mentioned underpressure, so that the above mentioned downstream end of the hub has a diameter increase relative to the remaining part of the above mentioned hub. The combination of relative profile of the downward widened end of the hub and the downward casing determines the overpressure in such a way that it reduces when the hub moves in the axial direction relative to the upstream casing.
Depending on the flow speed of the liquid the forces change in the flow direction and require a corresponding change in the oppositely directed dynamic pressure force. This is achieved by the axial movement of the impeller. In the known arrangement relatively great axial movements of the impeller are required in order to achieve the suspended condition of the blades. Moreover, the regulating conditions are relatively unstable.